Electric band type heaters are used to transfer heat to pipes and other cylindrical vessels. Band heaters are so called because they have a cylindrical shape, similar to a segment of a pipe, and are secured around the pipe to be heated with an inner face in heat transfer contact with the outer surface of the pipe. Band heaters are commonly used in the plastic molding industry to heat barrels and nozzles carrying molten plastic for injection molding or extrusion. The heat from the band heaters maintains the molten plastic temperature, and hence its fluid state and other essential characteristics, while it is being transported to the mold or extruder. When they are used in the plastic molding industry, band heaters may be exposed to molten plastic that leaks from the machinery as well as other potentially detrimental fluids in the factory environment. Therefore, it is advantageous to provide a band heater that resists infiltration by fluids.
Generally, band heaters are made up of an elongated electric heating element, an electrical insulator, and a multipart outer shell assembly that contains the heating element and insulation and provides for heat transfer from the band heater. Prior art band heaters have heat conducting shells made of two pieces, a pressure band and a base member. U.S. Pat. No. 3,872,281 describes such a prior art electric band heater. The pressure band is an elongated planar strip of conductive material such as steel that is narrower than the heating element/insulating material combination. The base member is a U shaped channel that receives the heating element and insulating material. The pressure band is placed on top of the insulative material such that it fits within the base member. The short, upstanding base member sides are then crimped down over the edges of the pressure band to retain the pressure band. This forms two pressure band to base member seams. The assemblage is then formed into a band configuration so that it can conform to a pipe when installed. Because a smooth, substantially continuous heat transfer surface is desirable, the pressure band and the pressure band to base member overlapped seams are often disposed around the radially outer face of the band heater. When installed, the band heater is placed on a pipe and a hose clamp is tightened about it. The crimped U channel sides are stiffer than the pressure band. When the hose clamp engages the pressure band between the crimped U channel sides the pressure band to base member seams tend to open. The open seams expose the heater band interior to environmental fluids. The seams are vulnerable to the ingress of molten plastic, airborne vapors or other destructive chemicals that may damage the heating element or insulation, unless each pressure band to base member seam is covered with a clamp.
If the band heater is constructed so that the pressure band to base member seams are disposed on the inner face of the band heater, the heat transfer surface is uneven and heat transfer will not be optimum because the heat transfer contact between the heater band and pipe is limited to the small areas of the crimped base member sides.
The present invention provides a new and improved electric band heater having a single piece outer heat conducting shell that provides a smooth substantially continuous heat transfer surface and is relatively resistant to the ingress of liquids.